FIELD OF THE INVENTION
The invention relates to a printing machine with a sheet-guiding surface, at least part of which is a cover surface of a respective pneumatic-flow box, such as a blowing box for blowing air and a suction box for suction air, the cover surface being formed with pass-through openings.
For reliable sheet guidance, it has become known heretofore to form parts of sheet-guiding surfaces as blowing/suction-box cover surfaces formed with pass-through or passageway openings. Especially in the sensitive curvature regions of a delivery drum of a chain delivery and of a preceding printing cylinder of the last printing unit, respectively, printed sheets of paper undergo very pronounced deflections. In this regard, it should be noted that, because of the extreme curvatures of the sheets of paper, the rear or trailing portion of the sheets readily departs from the sheet-guiding plane therefor. Damage to the printed image and to the sheet cannot be precluded in that case. At low conveying speeds, it should be noted that, in the region of the delivery drum wherein the sheet is taken over by the front or leading edge thereof by the delivery drum and wherein the front or leading edge is then passed, with a change of direction, beneath a rear or trailing region of the sheet, the rear or trailing edge of the sheet frequently flaps over in the direction of the axis of the delivery drum, and the rear or trailing edge of the sheet becomes damaged on elements of the delivery drum. If the printing machine is briefly stopped, the paper sheets in conventional suction box guides can tip over in the middle region of the delivery drum. Flapping-in or folding-in of the sheets at low speeds and with a brief stoppage of the printing machine also causes the sheets to wrap around the gripper bars of the delivery chains. The wound-up sheets of paper must be removed by tedious, time-consuming manual labor on the part of the operator. If they are not removed, they will continue to revolve with the gripper chain and can cause damage to the printing machine. Particularly at such critical deflection locations, it is especially important that the entire sheet-guiding region as much as possible, but at least the critical portion in the sheet guide, be reliably made. With heretofore known suction or blowing bars in the delivery drum region, only some regions are reliably guidable. Conventional suction/blowing boxes for sheet guiding, with a suction/blowing connection or nozzle optimally extending, as centrally as possible and with a large surface area, over the entire suction or blower region, are not readily usable, because of the tightness of space, especially in the region of the delivery drum. Suction/blowing boxes with a suction box region of narrow construction and with simple suction and blowing nozzles, respectively, which are undesirably disposed because of the tightness of space do not afford a safe and reliable sheet guidance over the entire suction box and or blowing box region, respectively, because of the high flow losses.
It is accordingly an object of the invention to provide a printing machine having a sheet-guiding surface, at least part of which is a cover surface of a respective pneumatic-flow or blowing/suction box, the cover surface being formed with pass-through openings, which ensure a reliable sheet guidance over the entire cover surface of the blowing/suction box.